1. Field of the Invention
The present invention relates to a novel information processing apparatus capable of writing, reading or erasing information by utilizing the technology of scanning type tunneling microscopes, scanning type atomic force microscopes, scanning type magnetic-force microscopes or the like, and to an information processing method employing the same.
2. Description Of the Related Art
In recent years, applications of memory elements have been of particular importance to the electronics industry relating to computers, devices related to them, video disks, digital audio disks or the like. Memory elements are being developed one after another. Examples of performance required of memory elements are generally as follows:
(1) High density and large recording capacity
(2) High response speed in recording and reproduction
(3) Low error rate
(4) Small amount of power consumption
(5) High productivity, and low cost.
In the past, semiconductor memories and magnetic memories in which magnetic bodies or semiconductors are used as materials are in the mainstream. However, with the recent advancements in laser technology, inexpensive, high-density recording media formed from optical memories using organic thin films, such as organic coloring matter or photopolymer, have appeared.
In the meantime, since a scanning type tunneling microscope (hereinafter abbreviated as STM) capable of directly observing the electronic structure of atoms on a surface of a conductor recently has been developed [G. Binning et al. Phys. Rev. Lett, 49, 57(1982)], it has become possible to measure a real space image with a high degree of resolution whether it is a single crystal or amorphous. Moreover, the STM has an advantage in that a specimen can be observed with low power without being damaged by electric current. In addition, since the STM can operate in air and can be used for various materials, a wide range of applications are expected.
Such STMs use the fact that when a voltage is applied between a metallic probe (probe electrode) and an electroconductive matter placed at a distance of approximately 1 nm, tunnel current flows. This current is very sensitive to changes in the distance between the probe and the electroconductive matter. Various information on real space electron clouds can also be read by scanning a probe in such a way that the tunnel current is maintained constant. At this time, resolution in an in-plane direction is approximately 0.1 nm.
Therefore, the application of the principle of an STM makes high-density recording and reproduction possible on an atomic order (sub-nanometer). In a recording and reproducing apparatus disclosed in, for example, Japanese Patent Laid-Open No. 61-80536, after atomic particles adsorbed on a surface of a medium are removed by an electron beam or the like, writing is performed and data is reproduced by a STM.
A method in which recording and reproduction are performed by the STM by using a material having a memory effect (electric memory effect) for voltage/current switching characteristics as a recording layer has been disclosed, an example of said material being a thin-film layer of .pi. electron type organic compounds, chalcogen compounds [Japanese Patent Laid-Open Nos. 63-161552 and 63-161553]. According to this method, if the bit size of recording is set at 10 nm, large-capacity recording and reproduction of as much as 10.sup.12 bits/cm.sup.2 is possible. A method of recording and reproduction employing the principles of an STM by using an electrically polarizable layer, for example, a thin-film layer of a ferroelectric substance, such as polyvinylidene fluoride, is disclosed in Japanese Patent Laid-Open No. 63-193349. According to this method, recording density of 10.sup.10 bits/cm.sup.2 can be achieved.
In the meantime, an atomic force microscope (hereinafter abbreviated as AFM) using STM technique has been developed [G. Binning et al., Phys. Rev. Lett, 56, 930(1986)], and information on irregularities on a surface can be obtained with AFMs in the same manner as with STMs. As AFMs are capable of measuring insulating specimens on an atomic order, advancements in such a microscope in the future are expected.
AFMs are constructed so as to support a probe by an elastic member and convert a displacement by the spring force of the elastic member. In an example of this construction, a probe is provided in the center of a beam supported at both ends or in the free end of a cantilever beam. Use of foils formed from Au, Ni, SUS, BeCuP, or the like is preferable for the materials of the beam. To prepare a very small beam used widely in micromechanics, preferable examples are SiO.sub.2 thin films and SiN thin films.
Examples of methods of detecting a displacement are a laser interference method (heterodyne detection), an electrostatic capacitance change detection method, and a method using an STM. Since an interatomic force is extremely small, a probe and an elastic support must be soft, but at the same time strong enough to withstand vibrations from outside.
An information processing apparatus using the AFM technique consisting of a scanning type capacitance microscope and a recording medium having silicon oxide and silicon nitride stacked on silicon (nitride-oxide-silicon, hereinafter abbreviated as NOS) has recently been developed [R. C. Barret et al. J.Appl.Phys., 70, 2725 (1991)]. The structure of NOS is shown in FIG. 11. NOS has the same layer construction as that of a memory used in EEPROMs whose upper electrode layer is removed. NOS is formed in such a way that an oxide film 112 for a tunnel barrier and a silicon nitride film 113 for trapping electrons are formed on a doped Si substrate 111. The SiO.sub.2 oxide film 112 has a thickness of approximately 10 to 50 .ANG., and the SiN film 113 has a thickness of approximately 500 .ANG.. FIG. 12 illustrates the construction of an information processing apparatus using an NOS recording medium. A probe electrode 1 is provided at an extreme point of the free end of a cantilever beam 2 which is an elastic supporting member.
The cantilever beam 2 is produced by using silicon crystal anisotropic etching technology and is formed from an SiO.sub.2 film. Such a production method is well known as a technique used in micromechanics [X. E. Petersen, Proc. IEEE 70, 420 (1982)].
A metallic film is vapor deposited in the form of wiring on the cantilever beam 2. The probe electrode 1 is connected to a capacitance sensor 122, a power supply or the like. A DC bias, a modulation signal, and a pulse voltage are applied to the recording medium and the probe electrode 1 during recording and reproduction.
Next, an explanation will be given of recording and reproduction performed by this information processing apparatus. To perform recording, an application of a pulse voltage greater than a threshold voltage causes electric charge from the silicon substrate 111 to tunnel through the SiO.sub.2 film 112 and to be trapped in an SiN film 113. As a result, information is recorded. To perform reproducing, an appropriate bias is applied to the recording medium and the probe electrode 1, a modulation signal is carried thereon, and capacitance changes during the scanning of the probe electrode 1 are detected. Since the capacitance between the probe electrode 1 and the recording medium changes depending upon whether a charge is trapped in the SiN film 113, information can be read out. To perform erasing, it is only necessary to apply a pulse voltage of the polarity opposite to that during recording.
Since the information processing apparatus performs recording and reproduction by a method employing an AFM having a resolution on an atomic and molecular scale, recording bits can be made much smaller, and it is thus easy to increase the capacity thereof. A major feature thereof is that a rewritable non-volatile memory is used.
FIG. 13 illustrates the construction of an information processing apparatus in which an STM is used.
Referring to FIG. 13, reference numeral 131 denotes a recording medium; reference numeral 132 denotes a substrate; reference numeral 133 denotes a substrate electrode; and reference numeral 134 denotes a recording layer. Reference numeral 1 denotes a probe electrode; reference numeral 135 denotes an XY stage; reference numeral 136 denotes a member for supporting the probe electrode; reference numeral 137 denotes a Z-axis linear actuator for driving the probe electrode in a Z direction; and reference numerals 138 and 139 denote linear actuators for driving the probe electrode along the X axis and along the Y axis, respectively.
Reference numeral 140 denotes an amplifier for detecting a tunnel current flowing from the probe electrode 1 through the recording layer 134 to the substrate electrode 133; reference numeral 141 denotes a compressor for converting a change in tunnel current into a value proportional to the distance between the probe electrode 1 and the recording layer 134; reference numeral 142 denotes a low-pass filter for extracting components of irregularities on a surface of the recording layer 134; reference numeral 143 denotes an error amplifier for detecting a discrepancy between a reference voltage V.sub.REF and an output from the low-pass filter 142; reference numeral 144 denotes a driver for driving the Z-axis linear actuator 137; reference numeral 145 denotes a driving circuit for controlling the positioning of an XY stage; and reference numeral 146 denotes a high-pass filter for separating data components.
Reference numeral 7 denotes a circuit for applying a pulse voltage for performing recording, reproduction and erasure between the probe electrode 1 and the electrode 133. When a pulse voltage is applied, a probe current sharply changes. Therefore, the driver 144 turns on a HOLD circuit (not shown) so that an output voltage becomes constant during the time that the pulse voltage is applied.
Another example of the information processing apparatus in which a STM is used is shown in FIG. 14 (Japanese Patent Laid-Open No. 1-134753).
This information processing apparatus has the following advantages as compared to the above-described information processing apparatus. First of all, high-speed scanning is possible. In a case where a two-dimensional scanning in X and Y directions is performed by a probe, if a scanning frequency is increased, high-order frequency components occurring at reciprocating scanning turns cause a scanning mechanism to resonate. However, when a recording medium 152 is located on a disk 151 as shown in FIG. 14, scanning is performed only radially, thereby substantially suppressing resonance and making high-speed scanning possible.
Secondly, the scanning mechanism is simple. Since the recording medium 152 rotates a constant number of rotations, if the probe can be controlled to move only radially, it can scan the entire recording surface. As compared to the two-dimensional scanning in X and Y directions, the scanning mechanism can be made more compact.
Next, the construction of an information processing apparatus in which both the AFM and STM techniques are used is shown in FIG. 15. This apparatus is designed to control the distance between a probe and a recording medium by a spring force of an elastic member supporting the probe by applying an AFM, as compared with the STM which controls the distance between the probe and the recording medium to be constant by using a tunnel current. The apparatus of FIG. 15 has an advantage in that a control system using a feedback circuit need not be provided for controlling the distance between the probe and the recording medium.
However, the prior art described above has problems as described below.
A first problem of the information processing apparatus using a NOS recording medium shown in FIG. 12 is that a threshold value during writing depends greatly upon the thickness of the SiO.sub.2 oxide film 112 for the tunnel barrier of a recording medium. Since the SiO.sub.2 oxide film 112 is extremely thin, it is difficult to precisely control the thickness of the film. For this reason, recording characteristics differ from recording medium to recording medium. A second problem of the information processing apparatus is that non-volatility deteriorates as the number of times writing is performed increases. Although non-volatility of this memory is retained by the SiO.sub.2 oxide film 112, the deterioration occurs if writing is performed many times since a trapped charge leaks because the SiO.sub.2 oxide film 112 is damaged. A third problem of the information processing apparatus is that since capacitance changes are used to reproduce information, the capacitance sensor 122 must be mounted on a probe electrode and a lock-in amplifier 121 must be connected thereto, thus enlarging the apparatus and making it complex.
In an information processing apparatus using a recording medium based on the principles of an STM and formed from a ferroelectric substance, information is written by applying an electric field based on the principles of an STM. Reading of information is performed by a complicated method in which a recording layer is heated by laser beam irradiation or high-frequency heating, the recording medium is activated pyroelectrically, and signals which occur are detected by an ultra-high resolution electrometer probe in a standard electrometer. It cannot be said that this is a simple method.
The information processing apparatus shown in FIG. 13 has a problem in that when a probe or a recording medium is scanned two-dimensionally in X and Y directions, resonance is likely to occur in the scanning mechanism if the scanning frequency increases.
The information processing apparatus shown in FIG. 14 has a problem described below. FIG. 16 illustrates the signal intensity spectrum of signals with respect to frequency during reproduction. Signals of frequency components lower than f.sub.0 are caused by gentle irregularities of a recording medium due to warping, strain or the like of the disk 151. Signals with f.sub.1 as the center are caused by irregularities on a surface of the recording medium. f.sub.2 indicates carrier wave components of recording data, and reference numeral 171 denotes a data signal band. f.sub.2 indicates a signal band which occurs due to the atomic and molecule array of the recording medium, and f.sub.T indicates a tracking signal.
The curved dashed line in this figure indicates a signal intensity spectrum when the probe is positioned at a given radius. The speed of the probe relative to that of the recording medium depends upon the position of the probe; the larger the radius, the higher the peripheral velocity of the probe becomes. Therefore, even if carrier waves or the tracking signal do not change, the components corresponding to the irregularities of the recording medium shift to a low or high frequency side in accordance with the position of the probe. As a result, the signal intensity spectrum of the signals reproduced from recorded information indicate a broad characteristic as a whole as indicated by the solid line. This makes it difficult to separate the tracking signal and the reproduced data signal, causing a decrease in the S/N ratio of the signals.
The information processing apparatus using AFM/STM shown in FIG. 15 has the same problem as that of the above-described apparatus in that the intensity spectrum of the signals expands broadly. FIG. 17 schematically illustrates a surface of the recording medium and a trace of a probe which scans the surface; FIG. 17(a) illustrates a case when the scanning speed is low, and FIG. 17(b) illustrates a case when the scanning speed is high. As can be seen in FIG. 17, the trackability of the probe onto the recording medium surface is affected greatly by the scanning speed. That is, the probe's signal intensity spectrum is broadly expanded as in the above-described example, causing a decrease in the S/N ratio of the signals.